This invention relates to a series of carboline derivatives, to processes for their preparation, to pharmaceutical compositions containing them, and to their use as therapeutic agents. In particular, the invention relates to carboline derivatives which are potent and selective inhibitors of cyclic guanosine 3',5'-monophosphate specific phosphodiesterase (cGMP-specific PDE) having utility in a variety of therapeutic areas where such inhibition is considered beneficial, including the treatment of cardiovascular disorders and erectile dysfunction.